The invention relates to a method for forming filter bags by joining together two layers of superimposed fibrous felt sheets by needling at selective areas, and also relates to felt filter bags formed by the needling method.
Forming filter bags from flat felt filter media has been conventionally done by cutting and folding, then machine stitching the edge seams together. One process comprises the steps of cutting out bag blanks by using templates, folding and sewing together the side seam, then sewing the bottom end closed and adding any finishing steps as desired. However, such felt filter bag production methods are relatively expensive, as stitching the side seam together can amount to between 25 and 75% of the total labor component in felt filter bag manufacture, depending on the size and shape of the bag. Also, undesired leakage of fluids can occur through the stitched seams of the filter bag.
A process which is used in the fabric product manufacturing industry for joining together fabric sheets is called mechanical interlocking, needle punching or needling, and consists basically of tucking a small bundle of individual fibers down through a carded batt of fibers in such large numbers of penetrations that a cohesive textile structure is formed. For example, U.S. Pat. No. 3,431,611 to Rentz has disclosed use of needling to produce a plurality of tubular channels on a layered substrate to form nonwoven shells for electric blankets. A pattern of longitudinal parallel lines is needled utilizing an appropriately patterned board, the resulting needled shells are then cut transversely into large articles, and the plurality of channels thus formed between the needles lines in each article receive the electrical cords of the blanket. Also, U.S. Pat. No. 4,955,116 to Hayamizu et al has disclosed a method for producing a tubular felt body which is needle punched along overlapping edges, in which at least a long sheet of felt material is overlapped by dislocating the sheets in width direction to form thin layer portions at both side edges of the overlapped sheets, and the overlapped thin layer portions are then needles together to form one tubular body. However, adapting such a method to the manufacture of felt filter bags would be complicated and uneconomic.
The method for forming filter bags from a fibrous or felt media material according to the present invention eliminates the folding step and the side seam stitching procedure of the prior art constructions, by utilizing a spatially oriented needling procedure for making seams which is similar to that used for making filter media sheets to replace the conventional stitching step for the seams. The present invention represents an important technical advance over the known prior art conditions, and will be clearly defined hereinafter.